This invention relates to rotary electric machines in general and to stator slot designs in particular. Radially oriented stator slots are used to mount stator bars in a stator frame surrounding a rotor. The stator bars in each slot are secured by a dovetail wedge which sustains forces from the stator bars. A conventional wedge is short in height, leaving the slot partially unfilled near the slot mouth. Due to the existence of such unfilled slots, the rotor body windage loss (or friction factor) on the rotor surface is rather high. Specifically, the rotor rotating-driven flow can penetrate into these unfilled slots so that the flow field in the stator-rotor gap is distorted and the friction factor on the rotor surface is increased significantly. Among various generator windage losses, the rotor body ranks the second largest.
The present invention relates to a reduction in the rotating body windage loss in rotary electric machines such as generators and induction motors.
We have discovered that the most effective way to reduce friction factor (or windage loss) is to redesign the dovetail wedge to fill all of the slot space. In this way, the flow in the air gap is similar to the flow between two concentric smooth cylinders, with the radially inner cylinder rotating. The comparison of friction factor profiles on the rotor surfaces has shown that with the proposed dovetail wedge, the rotor body windage loss can be reduced by about 26%.
In its broader aspects, the invention thus provides a rotating electric machine including a rotor and a stator surrounding the rotor in substantially concentric relation therewith to establish a gap therebetween, and wherein the stator is formed with a plurality of radially oriented slots, each slot receiving a stator bar held in place by a wedge, an arrangement to minimize rotor windage loss wherein each wedge substantially fills its respective slot such that an end surface of the wedge is substantially flush with adjacent surfaces of the stator.